I know how to create an alert that's triggered when a threshold is met.

My question pertains to creating alerts for a specific entity of a subset of items.

For example,

I want to create an alert that gets triggered if a specific employee, John Doe doesn’t work 40 hours in one week. I would want that alert email to tell me that specifically the employee, John Doe, did not work 40 hours.

Currently, I am only able to create an alert that gets triggered if ANY employee doesn’t work 40 hours and the alert email doesn’t specify which employee.

Basically, a separate alert email is sent to the admin for each employee who does work 40 hours in one week.

Does this need to be an alert that is triggered at a certain time, or would it work as a subscription?

You could set it up as a view which is either a user specific view automatically linked to a user or create separate views for each admin, then have this set up as a subscription at the end of the week with rules to not send if it is empty.

On point three, how large would the amount of employees be per user? Could you display all potential employees for an admin on a daily subscription at 8am for example?

If so, you wouldn't need to create individual views, but could do it with user specific views, so for example bring in the admin user name as a field against each employee, then set a filter for USERNAME() = [user name], that way the employee will only see the specific user for them, so will only get the subscription if one of their employees appears.